The fastest for me was to recognize that this is just question of finding 2^n > 500, where n is the smallest instance which makes the equation true.

2^9 = 512

9 X 2 (minutes) = 18.

1000----------500000
doubles in 2 minutes
1000*2^n=500000(approx)
2^n=500
n<9
now total minutes = 9*2=18 approx
it this correct answer?

If you remember what 2^9 is that's great, I choose to answer it with a chart:

Bacteria in thousands / minutes

1 / 0
2 / 2
4 / 4
8 / 6
16 / 8
32 / 10
64 / 12
128 / 14
256 / 16
512 / 18

It looks better on scratch paper when you make a T and use columns! Unless you memorize 2^9 you'll be doing this exercise in your head anyway. This doesn't take long and helps to visualize, I've found this method is a lot better when they throw things at you like, '6 minutes ago there were x bacteria, in how many minutes will there be y?' because you can see the timeline in front of you and that it makes sense. If you screw up by just one time interval you'll choose a wrong answer choice (since they always group their answer choices together on problems like this).